1. Field of the Invention
The present invention relates to accelerometers, and more particularly to the construction, sealing and packaging of monolithic vibrating beam accelerometers.
2. Description of the Prior Art
The prior art structure for a monolithic vibrating beam accelerometer (VBA) is illustrated in FIG. 1 as well as in FIG. 6 of U.S. Pat. No. 5,109,175 which issued to the same inventor as the inventor of the present invention. This figure describes a structure in which the vibrating beach sensing element 11, the accelerometer proof mass 12, the proof mass suspension system 13, and the structure mounting means 14 are all made from a single piece of quartz crystal. A key feature of this structure is that it is threedimensional whereby the inner sensing structure portion is of a much thinner section than the outer proof mass, suspension and mounting structure. As explained in U.S. Pat. No. 5,109,175, employing this three-dimension design allows the inner structure sensing requirements to be compatible with the outer structure mass, stiffness and strength requirements. This prior art design, however, lacks the improvement of an additional seal structure to the accelerometer.
Another prior art document, "A Reduced Hysteresis, Extended Range Quartz pressure Transducer", Ward, R. W. and EerNisse, E.P., IEEE 41st Annual Symposium on Frequency Control, May 27-29, 1987, describes a quartz pressure transducer which consists of a cylindrical center section and cylindrical end caps. Two key features of this transducer that will be used in the accelerometer design of the present invention are: (1) The end caps are secured to the center cylindrical section using a glass frit material. This frit is a low melting temperature glass-like material that accomplishes a vacuum-tight seal. (2) The center cylindrical section contains a resonating center section for which the resonance is maintained by the piezoelectric action of the quartz material and metal electrodes plated on the resonator. A key feature of the transducer design is that electrical communication to the internal electrodes is accomplished by a metallic conductive path which extends from the external surface of the device through the glass frit seal to the resonator. This feature of passing a conductive path through a seal joint will also be used in the present invention.
The monolithic quartz accelerometer structure of the prior art, however, is mounted into a machined metal housing that is, in turn, sealed by welding or soldering. The disadvantages of this method of construction are as follows: (1) The machined housing package is expensive. (2) Electrical communication from outside the package to the crystal is accomplished by electrical feed-throughs that must be glassed into the package at added expense. (3) Because the housing is metal, there is an expansion coefficient mismatch between the housing material and the quartz acceleration sensing structure. (4) A means to evacuate the housing must be provided which usually consists of an exhaust tube that is pinch-sealed. This also adds additional expense.